The invention relates to considering the propagation delay on a data link in a digital mobile communication system. The invention will be described using primarily the terms of the GSM system, but it can also be applied to other kind of digital mobile communication systems.
In a data call in a digital mobile communication system, the propagation delay is one of the parameters affecting connection quality. The extent of the propagation delay should affect the choice of system parameters. One such system parameter is the set value of a timer T1 associated with the GSM system RLP protocol (Radio Link Protocol). The timer is defined in the ETSI recommendation GSM 04.22.
Knowing the value of the propagation delay would contribute to the optimization of the timers of protocol software, particularly those of its layer 2. Connection-specific optimization of timer settings improves the reaction speed of protocol software for example in error situations when the integrity of user data has to be fixed by retransmission of either part or all of the frames in a transmission window. (A transmission window is the number of RPL frames the transmitting party can send without receiving acknowledgement from the receiving party.) If the timer is set too long, data transmission slows down, since potential errors are not corrected until the timer has expired. Setting the timer too short, again, results in a much more serious situation, since the transmitting party has to continuously request acknowledgements from the receiving party.
The problem is particularly perceivable when a connection includes a portion which causes much propagation delay, a satellite link, for example. In this case data calls may not succeed or the data transfer rate decreases significantly.
Still more significant variations in the propagation delay may occur in inter-BTS handover to a cell whose connection to the mobile switching centre is implemented via a satellite link. Such a situation is shown in FIG. 1, in which the mobile station MS moves along route 1 denoted by a broken line in such a way that the connection is initiated via a base station subsystem BSS1 which has a normal wired connection to the mobile switching centre, or, in short, to the centre, MSC. During the call, inter-BTS handover occurs to a base station system BSS2 whose connection to the centre MSC is via a satellite SAT.